1. Field of Invention
The present invention relates to a polarizing illumination device in which illumination light is polarized in a predetermined polarization direction, and to a projection display device provided with such a polarizing illumination device.
2. Description of Related Art
In order to obtain a bright display by improving light utilization efficiency, a projection display device employs an illumination device that converts light polarized in random directions (unpolarized light) into light polarized in one direction, and then uses the converted light. An example of using such a device for converting light polarized in random directions into light polarized in one direction is disclosed in Japanese Unexamined Patent Publication No. 7-294906. FIG. 8 is a plan view of an optical element for use in converting light polarized in random directions into light polarized in one direction. This optical element includes a polarizing beam splitter array 20 in which polarizing beam splitters 30 having polarization separating films 36 and prisms 40 having reflecting films 46 which are alternately bonded. The light incident surface of the polarizing beam splitter array 20 has a lens array 10 composed of a plurality of condenser lenses 11. On parts of the light emergent surface thereof, .lambda./2 phase plates 24 are selectively formed.
As shown in FIG. 8(A), light beams that are incident on the lens array 10 are split into a plurality of partial beams (intermediate beams) and are collected by the plurality of condenser lenses 11 constituting the lens array 10, and enter, as incident light including an s-polarized light component and a p-polarized light component, the polarizing beam splitters 30 that are located corresponding to the lens array 10. This incident light is first separated into s-polarized light and p-polarized light by the polarization separating film 36. The s-polarized light is almost perpendicularly reflected by the polarization separating film 36 that forms an angle of 45.degree. with the light incident surface, further perpendicularly reflected by the reflecting film 46 that forms an angle of 45.degree. with the light incident surface, and emerges from the prism 40. On the other hand, the p-polarized light passes unchanged through the polarization separating film 36, is converted into s-polarized light by the .lambda./2 phase plate 24, and is then emitted. Therefore, this optical element is a polarizing conversion element that converts most incident light beams polarized in random directions into s-polarized light and emits the converted light.
Ideally, all light beams that are incident on the lens array 10 should be collected by each of the condenser lenses 11 constituting the lens array 10, and should enter the polarizing beam splitters 30 corresponding to each of the condenser lenses 11. Light beams that are incident on the actual lens array 10, however, are not entirely collected, and some of the light beams enter the prisms 40, as shown in FIG. 8(B). This is because the size of light-source images (focal images) formed near the polarization separating films 36 is equivalent to the width of the polarization separating films 36, and because it is difficult to form light-source images that are sufficiently smaller than the width of the polarization separating films 36.
Such a light beam that is incident on the prism 40 is totally reflected by the reflecting film 46, and enters the polarizing beam splitter 30 disposed adjacent to the prism 40. The light beam that is incident on the polarizing beam splitter 30 is separated into s-polarized light and p-polarized light by the polarization separating film 36. The separated s-polarized light is reflected by the polarization separating film 36, is converted into p-polarized light by the .lambda./2 phase plate 24, and is then emitted. The p-polarized light passes through the polarization separating film 36, is reflected by the reflecting film 46 in the prism 40 disposed in the direction of transmission, and is then emitted. Therefore, the light beam that is incident on this optical element is emitted after it is not converted into a single light beam of s-polarized light, but into a light beam including a p-polarized light beam as well. The incident area of the polarizing conversion element is divided into effective incident areas EA and ineffective incident areas UA. The effective incident area EA means an incident area of the polarizing conversion element where an incident light beam is converted into desired polarized light and is emitted. The ineffective incident area UA means an incident area of the polarizing conversion element where an incident light beam is converted into undesirable polarized light and is emitted. Therefore, in this related art, the incident surfaces of the plural polarizing beam splitters 30 serve as the effective incident areas EA, and the incident surfaces of the plural prisms 40 serve as the ineffective incident areas UA.
When it is desired that only one type of polarized light be used, light which would enter such ineffective incident areas UA must be blocked by a polarizer or the like. That is, since the aforesaid p-polarized emergent light is not used in such a case, the light utilization efficiency deteriorates. In particular, a light source for use in an illumination device of a projection display device (a light source for emitting nearly-parallel light by using an electric-discharge lamp such as a metal halide lamp as a light-source lamp) provides a low light collecting ability near the optical axis thereof because the parallelism of light is low near the optical axis thereof, and it is difficult to reduce the size of light-source images near the polarization separating films 36. For this reason, there is a tendency for many light beams to enter the prisms 40. Moreover, since the light intensity is higher near the optical axis of the light source, the light utilization efficiency substantially decreases near the optical axis of the light source.